As is known, digitally addressed scan converters have been devised in the past for converting digital information stored in a computer, for example, into a video signal which can be displayed on a television receiving tube or used for other purposes. Such converters comprise a digitizing matrix which is flooded on one side with electrons. On the other side of the matrix is an electron target which can be scanned with an electron beam in the same way as the target of a vidicon tube. The digitizing matrix has a series of laser-drilled holes extending therethrough, the arrangement being such that when electrons from a flooding electron source on one side of the matrix pass through a laser-drilled hole, they will form one point of a scene on the target which can then be scanned with the electron beam.
In order to control the passage of electrons through the holes in the matrix and thus define a particular "picture" or scene on the target, it has been common to divide the matrix into a plurality of electrode units separated by air gaps, each unit comprising a layer of dielectric material having strips of conductive material thereon. The first electrode unit, for example, might have two spaced strips, the strips being perpendicular to those on the next succeeding unit. By applying digital signals to selected ones of the crossed strips on successive electrode units, electric fields can be generated in one or more quadrants of the matrix which will stop passage of electrons through laser holes drilled through that quadrant. The next two electrode units are the same as the first two except that each has four or 2.sup.2 crossed strips on a surface of a dielectric layer. The third pair of electrode units has 2.sup.3 strips deposited on spaced dielectric layers, and so on until, for a matrix having 2.sup.n .times. 2.sup.n holes therein, 2.sup.n crossed strips will be on opposite sides of the last electrode unit in the matrix; although in certain cases this number may be decreased by multiplexing techniques.
For example, to address a 512 .times. 512 element array, at least six electrode units are required. This requires laser drilling of 262,144 holes in each of the units, meaning that 160 hours of drilling time are required on the laser drill. The laser-drilled holes are about 0.001 inch in diameter on 0.003 centers; and the holes in the separated electrode units must be preferably aligned and remain in perfect alignment during the heating and cooling of the electron tube in which they are positioned during processing.
A disadvantage of the prior art system described above is that the dielectric used cannot be sufficiently thick, due to the large differential thermal expansions of silicon dioxide and silicon, causing spalling of the oxide. This results in the electrode units having a capacitance value which is too high to drive at a 10-megahertz rate without using pulses of approximately 100 amperes. Another disadvantage of the prior art system is that the assembly of the electrode units into a stack results in a stack which is approximately 2 inches thick. This results in a minimum tube thickness of about 3.5 to 4 inches.